An unidentified flying object
(usually abbreviated to UFO or U.F.O.) is any unusual apparent object or phenomenon in the sky whose cause cannot be identified
by the observer, or (in a narrower definition) by investigators; though in popular usage it more loosely means alien spacecraft, being one explanation (among several) offered for such sightings. Though
UFO sightings have occurred throughout history, modern interest in them dates from World War II, since when governments have investigated UFO reports, often
from a military perspective, and UFO researchers have investigated, written about and created organizations devoted to the
subject.

Studies have established
that the majority of UFOs are observations of some real but conventional object—most commonly aircraft, balloons, noctilucent clouds, nacreous clouds, or astronomical objects such as meteors or bright planets — that
have been misidentified by the observer as anomalies, while a small percentage of reported UFOs are hoaxes. However, after
excluding these incorrect reports, between 5% and 20% of the total remain unexplained, and so can be classified as unidentified
in the strictest sense. Many such reports have been made by trained observers such as pilots, police and the military; some
involve radar traces, so not all reports are visual. Proponents of an extraterrestrial hypothesis believe that these unidentified reports are of alien spacecraft,
though various other hypotheses have been proposed.

While UFOs have been the
subject of extensive investigation by various governments, and some scientists support the extraterrestrial hypothesis, few
scientific papers about UFOs have been published in peer-reviewed journals. There has been some debate in the scientific community about whether any scientific investigation into UFO sightings
is warranted.

The first widely publicized
U.S. sighting, reported by private pilot Kenneth Arnold in June 1947, gave rise to the popular terms "flying saucer" and "flying disc", of which the former is still sometimes used, even though
Arnold said the most of the objects he saw were not totally circular and one was crescent-shaped (see Kenneth_Arnold_UFO_sighting for details).

The term "UFO" was first
suggested in 1952 by Cpt. Edward J. Ruppelt, who headed Project Blue Book, then the USAF's official investigation of UFOs. Ruppelt felt that "flying
saucer" did not reflect the diversity of the sightings. He suggested that UFO should be pronounced as a word — you-foe.
However it is now usually pronounced by forming each letter: U.F.O. His term was quickly adopted by the United States Air Force , which also briefly used "UFOB". The Air Force initially defined
UFOs as those objects that remain unidentified after scrutiny by expert investigators, though today the term UFO is often
used for any unexplained sighting regardless of whether it has been investigated.

Because the term
UFO is ambiguous - referring either to any unidentified sighting, or in popular usage to alien spacecraft - and the public
and media ridicule sometimes associated with the topic, some investigators now prefer to use other terms such as unidentified
aerial phenomenon (or UAP)

Following the large U.S.
surge in sightings in June and early July 1947, on July 9, 1947, Army Air Force (AAF) intelligence, in cooperation with the FBI, began a formal investigation into selected best sightings with characteristics
that could not be immediately rationalized, which included Kenneth Arnold’s and that of the United Airlines crew. The AAF used
"all of its top scientists" to determine whether or not "such a phenomenon could, in fact, occur". The research was "being
conducted with the thought that the flying objects might be a celestial phenomenon," or that "they might be a foreign body
mechanically devised and controlled." Three weeks later in a preliminary defense estimate, the air force investigation decided
that, "This ‘flying saucer’ situation is not all imaginary or seeing too much in some natural phenomenon. Something
is really flying around."

A further review by the intelligence
and technical divisions of the Air Materiel Command at Wright Field reached the same conclusion, that "the phenomenon is something
real and not visionary or fictitious," that there were objects in the shape of a disc, metallic in appearance, and as big
as man-made aircraft. They were characterized by "extreme rates of climb [and] maneuverability," general lack of noise, absence
of trail, occasional formation flying, and "evasive" behavior "when sighted or contacted by friendly aircraft and radar,"
suggesting a controlled craft. It was thus recommended in late September 1947 that an official Air Force investigation be
set up to investigate the phenomenon. It was also recommended that other government agencies should assist in the investigation

UFOs are sometimes an element
of conspiracy theories in which governments are allegedly intentionally "covering
up" the existence of aliens or sometimes collaborating with them. There are many versions of this story; some are exclusive,
while others overlap with various other conspiracy theories.

In the U.S., an opinion poll
conducted in 1997 suggested that 80 % of Americans believed the U.S. government was withholding such information. Various
notables have also expressed such views. Some examples are astronauts Gordon Cooper and Edgar Mitchell, Senator Barry Goldwater, Vice Admiral Roscoe H. Hillenkoetter (the first CIA director), Lord Hill-Norton (former British Chief of Defense Staff and NATO head), the 1999 high-level French COMETA report by various French generals and aerospace experts, and Yves Sillard (former director of the French space agency CNES, new director of French UFO research organization GEIPAN).

It has also been
suggested by a few paranormal authors that all or most human technology and culture is based on extraterrestrial contact.